Polymeric foam sheet using ambient gas blowing agent via controlled expansion

ABSTRACT

An annular die designed to produce polymer foam using only one or more ambient gasses as a blowing agent includes an exiting channel with an exit having a cross-sectional area between about two and about ten times that of a smallest point within the exit channel. The section of the die from the smallest point to the exit is thermally isolated from the rest of the die, and the temperature thereof is independently controlled. In addition, the interior surface of the exit channel is coated with a friction-reducing coating.

BACKGROUND OF THE INVENTION

[0001] 1. Technical Field

[0002] The present invention generally relates to polymer foam sheetformation. More particularly, the present invention relates to polymerfoam sheet formation using only ambient gas blowing agents.

[0003] 2. Background Information

[0004] Polymer products produced by thermoforming of polymer sheet(e.g., polystyrene sheet) have proven useful in a great manyapplications. One of the major applications for this type of product isdisposable food packaging. Historically, there has been some marketplaceresistance to this product due to environmental concerns. One of thefirst major issues was concern of the destruction of the ozone layer dueto the release of the Chloroflourocarbons used as blowing agents.Chloroflourocarbons were eliminated and mostly replaced withhydrocarbons or hydrocarbon/carbon dioxide blends. Concern overlandfills then became the next major environmental issue addressed andhas been somewhat alleviated through ongoing recycling programs. Acurrent concern is, however, the use of hydrocarbon blowing agents.These gasses escape from the product over time, causing concern aboutozone generation at ground level. Hydrocarbon blowing agents alsopresent a practical problem to producers, as cumbersome environmentalpermits are required, as well as expensive pollution abatementequipment. Pollution abatement, however, addresses only thosehydrocarbons released at the plant location and does not address thehydrocarbon gasses that are shipped out of the production plant as partof the product and released later. In addition, these gasses are highlyflammable, resulting in expensive equipment and sensors designed forflammable environments, high insurance premiums, and most important,occasional worker injuries caused by fire.

[0005] In an effort to minimize the problems noted above with the use ofhydrocarbon blowing agents, many companies have resorted to usinghydrocarbon blends with ambient gasses, most significantly carbondioxide. However, commercial technology generally limits the amount ofcarbon dioxide that can be used to less than 50 mole percent of theblowing agent present at manufacture. Carbon dioxide has a very highvapor pressure compared with the hydrocarbon blowing agents. Ittherefore causes rapid expansion of the foam mass on exit of the diewhich results in sheet corrugation, thin gauge, irregular cell size,and/or poor surface appearance. Corrugation is defined as numerous gaugebands in the machine direction of foam sheet causing local gaugevariations of greater than 5%. Limiting the problems associated with theuse of carbon dioxide limits the amount that can be successfully used inmanufacture without compromising quality or increasing product weight.Any attempts to use only carbon dioxide as a blowing agent have resultedin product that is of either inferior quality or more than 10% greaterin weight compared to product made using at least a portion ofhydrocarbon blowing agents. These problems are magnified for otherambient gasses, as they have vapor pressures much higher than carbondioxide.

[0006] An additional problem found with the use of ambient gasses is thelack of post expansion when thermoforming the sheet. The permeation rateof hydrocarbons through polystyrene is much slower than that of ambientgasses. As a result, upon aging of the sheet for a number of days, thepartial pressure inside the foam sheet is significantly greater than oneatmosphere. Thus, by the ideal gas law, the sheet can be expected toexpand between 50% and 120% when heated to the glass transitiontemperature. Without this increase in partial pressure, only thermalexpansion effects of 10% to 40% gauge increases are realized. Thisexpansion is necessary to produce the low densities required of thefinal product. It is known that the strength-to-weight ratio improvesfor foam as the density is reduced. Therefore, without this expansion,the product would be weaker and require more weight to compensate, whichagain has a severe negative economic impact. One potential solution isto reduce the density of the sheet such that further density reductionrequirements at forming are not needed. This solution requires moreblowing agent, which exasperates all of the negatives associated withthese ambient blowing agents already described.

[0007] There exists, therefore, a need to produce polymer foam sheetusing only ambient gasses that has little or no corrugation, uniformcell structure, and low density. Such sheet could then be thermoformedinto the final product desired without either reduction in quality orincrease in product weight.

SUMMARY OF THE INVENTION

[0008] Briefly, the present invention satisfies the need to producepolymer foam using only one or more ambient gasses as blowing agents byproviding an annular die designed to address the problems resulting fromtheir exclusive use.

[0009] In accordance with the above, it is an object of the presentinvention to provide an annular die for producing polymer foam usingonly one or more ambient gasses as blowing agents.

[0010] The present invention provides, in a first aspect, a method ofproducing polymer foam. The method comprises heating a polymer resin toits melt temperature, selecting at least one blowing agent consisting ofat least one ambient gas, combining the heated polymer resin with theblowing agent(s) to create a mixture, and extruding polymer foam sheetfrom the mixture comparable in quality to that obtainable withhydrocarbon blowing agents.

[0011] The present invention provides, in a second aspect, an annulardie for producing polymer foam. The annular die comprises an exitingchannel having an exit with a cross-sectional area larger than at leastone point within the exiting channel.

[0012] These, and other objects, features and advantages of thisinvention will become apparent from the following detailed descriptionof the various aspects of the invention taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a cross-sectional view of a representative prior artannular die assembly.

[0014]FIG. 2 is a cross-sectional view of an annular die in accordancewith the present invention wherein constraining geometry is an integralpart of the die lips.

[0015]FIG. 3 is a cross-sectional view of an annular die in accordancewith the present invention wherein constraining geometry is coupled tothe die and the transition angle is about 10°.

[0016]FIG. 4 is a cross-sectional view of an annular die in accordancewith the present invention wherein constraining geometry is coupled tothe die and the transition angle is about 90°.

[0017]FIG. 5 is an exploded view of one section of the annular die ofFIG. 4.

[0018]FIG. 6 is a cross-sectional view of a prototype annular die usedto produce the results of Example 1.

DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention constrains the growth rate of polymericfoams such that foam sheet can be produced at low density with nocorrugation, uniform cell size, and good surface appearance. An annularconstraining geometry is added to typical annular dies by eitherincorporating the constraining geometry as part of the die lips oradding an external constraining geometry section to the die. Theconstraining geometry allows the polymer mass containing blowing agentto partially expand before leaving the die structure. A central problemin producing foam using such a die is that pre-foaming occurs, whichtypically results in poor surface quality and irregular cell size.Previous limitations of such a die are overcome by thermally isolatingthe constraining section of the die from the rest of the die structure,and independently controlling the temperature in that section.Additionally, an interior surface coating further reduces friction. Inthis way, the frictional characteristics of the die and the rate ofexpansion are controlled such that a foam sheet with smooth surface anduniform cell structure is obtained. Additionally, since the foamstructure is substantially reduced in density before exiting theconstraining geometry, the further rate of foam expansion is reduced andcorrugation is avoided. Low density foam sheet can therefore be producedsuitable for thermoforming into final products that have quality andpart weights comparable in quality to current products on the marketusing only ambient gasses as blowing agents (i.e., carbon dioxide,nitrogen, and argon).

[0020] Before describing the present invention, it will be useful tounderstand the typical construction of a conventional annular die. FIG.1 is a cross-sectional view of such a die. A die adaptor 100 is used toconnect the die 102 to a typical foam extruder. Polymer passes throughthis adaptor into the body 104 of the die. The die body contains meansfor mounting an interior male die lip 106. The mounting means maycomprise either a spider or breaker plate assembly. In FIG. 1, a breakerassembly 108 is represented. The function of this assembly is to providea means of mounting the interior die lip while allowing the polymer tosimultaneously pass through. The interior die lip and an exterior dielip 110 are attached to the die and define an opening 112 for thepolymer to exit. The opening may be adjusted by moving either die lipappropriately. Additionally, means of centration are provided, in thiscase four centering bolts 114 (since FIG. 1 is a cross-sectional view,only two bolts are shown) allow the male die lip to be moved such thatextrudate is uniform around the entire circumference of the die. Variousinstruments such as thermocouples and pressure transducers may be usedto monitor the performance of the process and are mounted in theprovided ports 116.

[0021] Typical of annular dies, the minimum cross-sectional opening ofthe exit channel is at the actual exit of the die (here, opening 112,also referred to as the die gap). When producing a foam product, diepressures are maintained such that the blowing agent remains insolution. Most desired is a halo appearance at the die exit indicatingthat foaming is occurring after exiting of the die. This typicallyresults in a uniform cell structure and uniform surface appearance.Should the die pressure not be high enough to keep the blowing agent insolution, a condition known as pre-foaming will occur. The extrudatewill begin foaming within the die body. The resultant foam will becharacterized as having a very poor surface appearance and poor cellsize distribution. Product made from such foam is inferior in qualityand is generally rejected by the customer.

[0022] The die pressure needed to maintain the blowing agent in solutiondepends on the blowing agent selected. The vapor pressure and solubilityof the gas in the polymer will determine the needed die pressure toprevent pre-foaming. In the case of polystyrene, for example, oneblowing agent that has proven to work well is pentane. At aconcentration of about 5%, pentane will typically yield a uniform foamwith a specific gravity of about 0.1 g/cc (i.e., grams per cubiccentimeter). The die pressure required to keep this gas in solution isless than 140 bar, which allows a die opening large enough to avoidcorrugation. Corrugation occurs when the rate of foam growth exceeds theavailable geometry. Specifically, the circumference of the growing foamexceeds the circumference geometrically available and the additionallength forms a sinusoidal pattern about the center of radial growth. Thenegative impact of this phenomenon is that the foam will be thicker andthinner in spots across the sheet width and the product made with suchfoam will have inferior physical properties. Since product thickness,width and basis weight are controlled variables, both the amount of gasneeded to attain the desired density reduction (and thereby thickness)and the machine direction speed are fixed. In addition, since foamgrowth is three-dimensional, the die must be smaller in circumferencethan the final width of the sheet produced. These constraints thereforeallow only limited freedom in geometric configuration. Ideally, thegrowth in all three dimensions is at a rate of the cube root of theexpansion ratio, where the expansion ratio is defined as the finaldensity of the foam divided by the density of the unexpanded melt. Suchgrowth would result in nearly spherical cells and equal strength of theproduct in each dimension.

[0023] In practice, however, limitations result in growth that isgenerally unbalanced in the three dimensions. If the die gap is toosmall, for example, the thickness of the product cannot be achieved andextra growth will be forced into radial growth, which can causecorrugation. Also, if the ratio of the width of the sheet (which isgenerally determined by the circumference of the cooling mandrel usedafter sheet formation) to the circumference of the die is too small,corrugation will also occur. It is therefore desired to keep this ratiohigh enough so that the melt cannot be further stretched withouttearing. For polystyrene foams, this ratio is generally between 3 and 5.

[0024] The problem with using ambient gasses to produce polymer foamsheet is therefore complex. These gasses have very high vapor pressuresand very low solubility in most polymers. A high die pressure istherefore required to prevent pre-foaming at the gas concentrationsrequired to make low-density foam. For the purpose of this invention,low density foam is defined as foam with a specific gravity less than0.15 g/cc. A high die pressure requires a small die gap, which thencauses the sheet to corrugate severely and limits the thickness thesheet can attain. Additionally, due to the high vapor pressure of thesegasses, the foam reaches its final density very quickly after exitingthe die. Again, corrugation will occur as the circumference of thegrowing sheet is at nearly the final sheet width while the geometricconstraint is still near the circumference of the die. Again, aspreviously discussed, the additional length must be accommodated as asinusoidal pattern about the center of the circle defining the radialgrowth. As a result, using ambient gasses exclusively has alwaysresulted in either a product that was inferior due to pre-foaming orcorrugation, or both, or a product of limited density reduction producedby limiting the amount of ambient blowing agent to avoid the problemspreviously described.

[0025] The present invention solves these problems through a reductionin the growth rate of the foam after exiting the die by allowing somegrowth within the die. By carefully controlling both the amount ofgrowth and the rate of growth, a uniform cell structure can be achieved.In addition, by controlling the frictional characteristics of thepolymer-die interface, either through application of coatings and/or viatemperature control of the die, a uniform smooth surface can bemaintained on the foam product. By using a die designed for thispurpose, foam sheet can be made using only ambient gasses that iscomparable in density and quality to sheet made using conventionalhydrocarbon-based blowing agents. Typical density for such aconventional product would be about 0.05 to about 0.15 g/cc, typicalcell size would range from about 0.05 to about 1.0 mm, and a typicalthickness would range from about 0.75 to about 6 mm. Additionally, thesurface would be uniform in appearance and there would be no corrugationgauge bands in the product. Therefore, polymer foam sheet produced inaccordance with the present invention preferably has a thickness ofbetween about 0.75 mm and about 6 mm gauge variation across a widththereof of less than about 5%, and a uniform appearance.

[0026]FIG. 2 is a cross-sectional view of one example of an annular die200 in accordance with the present invention, in which the constraininggeometry is an integral part of the die lips. Not shown are the adaptor,die body, male lip mounting, concentration means, and instrumentationports which remain unaltered from FIG. 1. In FIG. 2, the male die lip202 and female die lip 204 depicted have been greatly altered, comparedto FIG. 1. The die lips establish a minimum die opening 206 within anexiting channel 208 in the body of the die, generally in the range ofabout 0.25 mm to about 1.0 mm, depending on the die pressure needed tokeep the blowing agent in solution. The lips then diverge to form anexit opening 210 that is between about 2 to about 10 times greater incross-sectional area than the cross-sectional opening defined by theminimum opening. For purposes of this description, the exit point of thedie is defined as the point at which the foam sheet leaves the diesurface. This can occur within the exiting channel if the sheet to bemade is thinner than the final exit point. The angle 212 at which thedie lips diverge will depend on the polymer/blowing agent mixture to befoamed, as will the exit length 214. For polystyrene foamed with carbondioxide, for example, a divergence angle of about 3° and an exit lengthof about 25.4 mm has been found useful. It should be noted that thedivergence over the exit length can be linear or some other geometry.Usefulness of this concept is not limited to linear geometry. Anadditional design parameter is the die exit angle 216, which is shown tobe about 34° in FIG. 2, but may range from 0 to about 90°.

[0027] By using the above geometry, the foam growth rate can becontrolled so as to form uniform cells in a sheet without corrugation.Without other die modifications, however, the surface quality of thefoam would still be poor. To attain a uniform surface of goodappearance, the frictional properties of the foam/die lip interface arealso modified. This may be accomplished by, for example, applyingcoatings on the inner surface of a portion or all of the exitingchannel, such as, for example, a composite of nickel and eithertetrafluoroethylene fluorocarbon polymer or fluorinatedethylene-propylene (e.g., TEFLON by du Pont), titanium nitride, tungstencarbon carbide, or other similar coatings with good slipcharacteristics. Another example of modifying frictional properties asshown in FIG. 2, is by temperature control of the expanding portion ofthe exit channel beyond minimum opening 206. In FIG. 2, temperaturecontrol is accomplished by circulating tempered oil, for example,through cooling channels 218 provided. An external cooling/circulationunit 219 can be used to temper the die exit geometry to the propertemperature and circulate the oil via coupled conduits (shown inphantom). For polystyrene foams, for example, a temperature of about 30C. to about 40 C. has been found to be particularly useful. It is alsopreferable to have independent control of the male and female die liptemperatures, and for this reason, it is useful to have separate coolingand circulation units for each.

[0028] The temperature needed to produce a uniform skin of good qualityis, however, generally low enough to freeze the extrudate within theexiting channel over a short period of time. Thermal breaks 220 solvethis problem by minimizing heat transfer in the die at the point of theminimum opening 206, and thereby preventing the extrudate from freezingin the die.

[0029]FIG. 3 is a cross-sectional view of another example of an annulardie 300 in accordance with the present invention. In FIG. 3, theexpansion geometry 302 is attached to existing male die lip 304 andexisting female die lip 306. Unlike die 200, where the exit geometry wasan integral part of the lips, in FIG. 3, the exit geometry is coupled tothe die at the lips. Exit female die lip portion 308 is coupled toexisting female lip 306 and exit male die lip portion 310 is coupled toexisting male die lip 304. As with FIG. 2, cooling channels 312 fortempering of the exit lips are provided. Inserts 314 and 316 are oneexample of how to achieve the cooling channels. Similarly, thermalbreaks 318 between the new die lip portions and the existing die lipsare also provided. As shown in FIG. 3, the divergence angle 320 is about10° with a uniform transition from the point 322 of smallestcross-sectional area in the exit channel. The exit angle 324 of thisexample is shown to be about 45°.

[0030]FIG. 4 is a cross-sectional view of the die lip portion 400 ofanother example of an annular die in accordance with the presentinvention. In FIG. 4 as in FIG. 3, the female exit lip 402 is coupled tothe existing female die lip 404, and the male exit lip 406 is coupled tothe existing male die lip 408. Again, cooling channels 410 and a thermalbreak 412 are provided, along with inserts 411 and 413. In the exampleof FIG. 4, the divergence angle 414 is about 0° to an opening 416 (seeFIG. 5), however a transition angle 418 exists wherein the die openingtransitions to the divergence channel. The transition angle in the FIG.4 example is 180°, but may vary from about 15° to about 180°.

EXAMPLE 1

[0031] Experimental sheet foam was manufactured using a two-inch primaryby 2.5 inch tandem extrusion line, under the following conditions:Temperature: Primary Extruder Zone 1 177° C. Primary Extruder Zone 2199° C. Primary Extruder Zone 3 215° C. Primary Extruder Zone 4 221° C.Primary Extruder Zone 5 223° C. Crossover Zone 6 234° C. Crossover Zone7 218° C. Seal Zone 8 101° C. Secondary Extruder Zone 9 121° C.Secondary Extruder Zone 10 107° C. Secondary Extruder Zone 11 102° C.Secondary Extruder Zone 12 110° C. Die Zone 13 142° C. Die Zone 14 132°C. Coupling Melt 222° C. Die Melt 133° C. Pressures: Blowing AgentCompressor 310.3 Bar Injection Port 227.6 Bar 2 Inch Extruder 234.5 BarCrossover 194.5 Bar Die  59.3 Bar Drive Conditions: Primary Speed   75rpm Primary Current   30 amps Secondary Speed 24.8 rpm Secondary Current  18 amps Formulation: Polymer Type Polystyrene (Dow STYRON 685D)Polymer Rate 25.5 kg/hr Nucleator Type Talc Concentrate (50%) NucleatorRate 0.36 kg/hr Blowing Agent Type Carbon Dioxide Blowing Agent Rate0.77 kg/hr Miscellaneous: Date of manufacture Mar. 19, 1996 Time ofSample 12:00 pm Test Data: Average Thickness 9.15 mm Maximum Deviation0.19 mm Percentage Deviation 2.1% Sample Density 0.088 g/cc Sample CellSize 0.87 mm

[0032] The sample of Example 1 was produced with the annular die 600 ofFIG. 6. A male die lip 602 and female die lip 604 create an opening 606.The divergence angle 608 of this die was about 3°, the exiting channel610 had a length of about 25.4 mm and the exit angle 612 of the die wasabout 34°. Annular die 600 had exterior (female die) lip air coolingmeans 614 only, which kept the die at about 60° C. No means of coolingthe male die lip was provided on this prototype die. In general, thesection of the exiting channel from the smallest cross-sectional pointto the exit is preferably kept at a temperature of between about 15° C.and about 95° C., and more preferably at a temperature of between about25° C. and about 60° C., this temperature being determined subjectivelyby the best surface appearance. In addition, the foam producedpreferably has a specific gravity of between about 0.05 g/cc and about0.15 g/cc, and an average cell diameter of about 0.05 mm to about 1 mm.

[0033] As can be seen from the data above, the sample produced has adensity typical of foam sheet. The cell size and thickness were bothextraordinarily large for foam made using only carbon dioxide,indicating that the constraining section of the die did indeed inhibitthe rate of cell growth. The sample showed no visual signs ofcorrugation as evidenced by a sample thickness deviation of less than3%. The exterior skin of this product was smooth and uniform while theinterior skin was rough and showed signs of melt fracture. Thisindicates that tempering of the constraining surface produces a uniform,saleable skin surface. Die pressures rose slowly during the trial as thecool lips excessively cooled the die itself, indicating the need for athermal break between the cooled constraining section of the die and therest of the die and die body.

[0034] While several aspects of the present invention have beendescribed and depicted herein, alternative aspects may be effected bythose skilled in the art to accomplish the same objectives. Accordingly,it is intended by the appended claims to cover all such alternativeaspects as fall within the true spirit and scope of the invention.

1. A method of producing polymer foam, comprising: heating a polymerresin to a melt temperature therefor; selecting at least one blowingagent consisting of at least one ambient gas; combining the heatedpolymer resin with the at least one blowing agent to create a mixture;and extruding polymer foam from the mixture comparable in quality tothat obtainable with hydrocarbon blowing agents.
 2. The method of claim1, wherein the extruding comprises guiding the mixture through anexiting channel to an exit with a cross-sectional area larger than atleast one point within the exiting channel.
 3. The method of claim 2,wherein the cross-sectional area of the exit is at least about twice aslarge as that of the at least one point.
 4. The method of claim 2,wherein the extruding further comprises reducing friction within atleast a portion of the exiting channel.
 5. The method of claim 4,wherein the exiting channel comprises a first portion from an entranceto a point having a smallest cross-sectional area and a second portionfrom the point having the smallest cross-sectional area to the exit, andwherein the reducing comprises controlling a temperature of the secondportion.
 6. The method of claim 5, wherein the controlling compriseskeeping the second portion at a temperature of between about 15° Celsiusand about 95° Celsius.
 7. The method of claim 6, wherein the keepingcomprises keeping the second portion at a temperature of between about25° Celsius and about 60° Celsius.
 8. The method of claim 5, furthercomprising at least partially thermally isolating the first portion fromthe second portion.
 9. The method of claim 8, wherein the at leastpartially thermally isolating comprises locating at least one air gapbetween the first portion and the second portion.
 10. The method ofclaim 4, wherein the reducing comprises coating the at least a portionof the exiting channel with a friction-reducing substance.
 11. Themethod of claim 10, wherein the coating comprises coating the at least aportion of the exiting channel with titanium nitride.
 12. The method ofclaim 10, wherein the coating comprises coating the at least a portionof the exiting channel with tungsten carbon carbide.
 13. The method ofclaim 10, wherein the coating comprises, coating the at least a portionof the exiting channel with a composite comprising nickel and one oftetrafluoroethylene fluorocarbon polymer and fluorinatedethylene-propylene.
 14. The method of claim 1, wherein selecting the atleast one blowing agent comprises selecting from among carbon dioxide,nitrogen and argon.
 15. The method of claim 1, wherein the extrudingcomprises extruding polymer foam from the mixture having a specificgravity of between about 0.05 g/cc and about 0.15 g/cc and an averagecell diameter of about 0.05 mm to about 1 mm.
 16. The method of claim15, wherein the extruding comprises extruding polymer foam sheet fromthe mixture having a thickness of between about 0.75 mm and about 6 mm.17. The method of claim 16, wherein the extruding comprises extrudingpolymer foam sheet from the mixture having less than about 5% gaugevariation across a width thereof.
 18. An annular die for producingpolymer foam, comprising an exiting channel having an exit with across-sectional area larger than at least one point within the exitingchannel.
 19. The annular die of claim 18, wherein the cross-sectionalarea of the exit is at least about twice as large as that of the atleast one point.
 20. The annular die of claim 18, wherein an exit angleof foam sheet produced with the annular die is between 0° and about 90°.21. The annular die of claim 18, wherein the exiting channel comprises afirst portion from an entrance to a point having a smallestcross-sectional area and a second portion from the point having asmallest cross-sectional area to the exit.
 22. The annular die of claim21, further comprising a thermal break between the first portion and thesecond portion.
 23. The annular die of claim 22, wherein the thermalbreak comprises at least one air gap.
 24. The annular die of claim 21,wherein the first portion and the second portion are integrated.
 25. Theannular die of claim 21, wherein the second portion is coupled to thefirst portion.
 26. The annular die of claim 18, further comprising afriction-reducing coating on at least a portion of an inner surface ofthe exiting channel.
 27. The annular die of claim 26, wherein thefriction-reducing coating comprises titanium nitride.
 28. The annulardie of claim 26, wherein the friction-reducing coating comprisestungsten carbon carbide.
 29. The annular die of claim 26, wherein thefriction-reducing coating comprises a composite comprising nickel andone of tetrafluoroethylene fluorocarbon polymer and fluorinatedethylene-propylene.
 30. The annular die of claim 18, wherein atransition angle is between about 15° and about 180°.
 31. A system forproducing polymer foam, comprising: an annular die for producing polymerfoam, comprising an exiting channel having an exit with across-sectional area larger than a point within the exiting channelhaving a smallest cross-sectional area, wherein the exiting channelcomprises a first portion from an entrance to the point and a secondportion from the point to the exit; and means for temperature regulatingthe second portion.
 32. The system of claim 31, wherein the means fortemperature regulating the second portion comprises at least one channelin the annular die for circulating a liquid.
 33. The system of claim 31,wherein the cross-sectional area of the exit is at least about twice aslarge as that of the point.
 34. The system of claim 31, wherein an exitangle for the annular die is between about 0° and about 90°.
 35. Thesystem of claim 31, further comprising a thermal break between the firstportion and the second portion.
 36. The system of claim 35, wherein thethermal break comprises at least one air gap.
 37. The system of claim31, wherein the first portion and the second portion are integrated. 38.The system of claim 31, wherein the second portion is coupled to thefirst portion.
 39. The system of claim 31, further comprising afriction-reducing coating on at least a portion of an inner surface ofthe exiting channel.
 40. The system of claim 39, wherein thefriction-reducing coating comprises titanium nitride.
 41. The system ofclaim 39, wherein the friction-reducing coating comprises tungstencarbon carbide.
 42. The system of claim 39, wherein thefriction-reducing coating comprises a composite comprising nickel andone of tetrafluoroethylene fluorocarbon polymer and fluorinatedethylene-propylene.
 43. The system of claim 31, wherein a transitionangle for the annular die is between about 15° and about 180°.